Modern wireless communications systems for speech communications are commonly implemented using a speech coder operating at a fixed bit rate, a channel coder operating at a fixed bit rate, and a modulator operating at a fixed modulation format. These systems ordinarily rely on specific, modestly changing channel conditions, however, in a typical system, channel conditions are continuously changing and may experience dramatic variation. A problem with such systems is a failure to allocate optimal bit rates and modulation strategies for controlling the system elements based on current channel conditions.
For example, when an analog channel for speech communications has very little noise, existing systems do not take advantage of channel conditions. Alternatively, when the same channel degrades because of the presence of noise, existing systems do not compensate for such degrading channel conditions.
Typical network communication systems also experience changing channel conditions. For example, Internet packets (e.g., a set of bits of a predetermined size) may be transmitted via Internet Protocol. Systems using Internet Protocol typically deliver a packet without error and fail to deliver a packet with errors. Packets may also be delivered "late". Additionally, an intelligent router conveys packets via different communications paths based on system congestion. Speech communications systems using a network are designed to operate under modestly changing channel conditions. However, since channel conditions may change dramatically, a problem with such systems is an inability to adjust operating parameters when packets are delayed, lost, or out of sequence.
Additionally, current systems fail to change operating parameters when the available channel capacity fails to support the modulation rate. So, another problem with existing systems is a failure to reduce the bit-error-rate for the system.
Another problem with existing systems is that speech quality degrades because of lower bit rate speech coding. Current systems fail to provide a graceful degradation to lower bit rate speech coding when noise in the channel causes synthesized speech to be perceived as noisy, distorted, and unintelligible speech, instead of synthesized speech having less natural speech quality.
Another problem with existing wireless systems is adjusting power levels for a transmitter to compensate for noise in a channel. For example, when excessive noise is determined in a receiver, the receiver requests that the transmitter raise its power to overcome the noise in the channel. This is a problem because raising the power for the transmitter may not improve the quality of the speech synthesized at the receiver.
Thus, what is needed are a system and method to dynamically control operating parameters for a transmitter and a receiver based on changing channel conditions. What is also needed are a system and method for dynamically allocating bits for error coding based on noise level in a communication channel. What is also needed are a system and method that takes advantage of good channel conditions, and compensates for poor channel conditions to maintain voice quality.